K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors

After passage through matter, the energy spectrum of a polychromatic beam of x-rays contains valuable information about the elemental composition of the absorber. Conventional x-ray systems or x-ray computed tomography (CT) systems, equipped with scintillator detectors operated in the integrating mode, are largely insensitive to this type of spectral information, since the detector output is proportional to the energy fluence integrated over the whole spectrum. The main purpose of this paper is to investigate to which extent energy-sensitive photon counting devices, operated in the pulse-mode, are capable of revealing quantitative information about the elemental composition of the absorber. We focus on the detection of element-specific, K-edge discontinuities of the photo-electric cross-section. To be specific, we address the question of measuring and imaging the local density of a gadolinium-based contrast agent, in the framework of a generalized dual-energy pre-processing. Our results are very promising and seem to open up new possibilities for the imaging of the distribution of elements with a high atomic number Z in the human body using x-ray attenuation measurements. To demonstrate the usefulness of the detection and the appropriate processing of the spectral information, we present simulated images of an artherosclerotic coronary vessel filled with gadolinium-based contrast agent. While conventional systems, equipped with integrating detectors, often fail to differentiate between contrast filled lumen and artherosclerotic plaque, the use of an energy-selective detection system based on the counting of individual photons reveals a strong contrast between plaque and contrast agent.     

Noise characteristics of a microchannel plate x-ray image intensifier

The noise performance of an experimental microchannel plate x-ray image intensifier has been evaluated. The intensifier, constructed for use with photons of energies between 20 and 150 keV, uses an MCP as the photon-to-electron converter. The influence of noise was determined by analysis of the optical-density fluctuations of a photograph of the viewing screen of the intensifier when the conversion layer was exposed to between 1 and 60 mR. Additionally, the contrast-detail performance of the experimental device was determined. The influence of both stochastic noise, due to quantum mottle and pulse-height variations, and structural noise, due to fluctuations in inherent gain from point to point, have been considered by using a model that adds these components.     

Image resolution of a microchannel plate x-ray image intensifier

The resolution has been evaluated for an experimental x-ray image intensifier employing three microchannel plates (MCPs) as the photon absorber and electron multipliers. The line spread function (LSF) was measured and used for determination of the modulation transfer function (MTF). The MTF was found to be independent of the incident photon energy from 20 to 150 keV. An additional measurement using a lead-bar test pattern showed that the resolution exceeded 7 line pairs (lp)/mm. The factors influencing the resolution capabilities of the intensifier are discussed. The resolution is limited primarily by the 53-micron center-to-center separation of the channels of the MCPs.     

DQE analysis on a dual detector phase x-ray imaging system

This study presents the characterization results of a newly developed dual detector in-line phase x-ray imaging prototype. Comparison of modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) for both detectors was conducted when they worked in the dual detection mode, in which two images are acquired simultaneously at a single exposure. The MTFs of the two detectors are almost identical, showing that the blurring caused by detector1 does not significantly weaken the resolving power of detector2. With a 40 kVp and 4 cm thick BR-12 phantom filtered x-ray beam, the transmittance of detector1 was measured to be 32%. The characteristic response and DQE of the two detectors almost coincide, showing that the two detectors have similar imaging performance under the imaging conditions of this study. The DQE of detector2 at the different source to detector distances (SID) also demonstrate a high level of agreement, implying that the reduced exposure level caused by elongated SID did not degrade the performance significantly. The study validated the design of the dual detection configuration for phase x-ray imaging, which has the potential for improving the accuracy of diagnostics at clinically acceptable radiation doses.     

A multiple detector array helical x-ray microtomography system for specimen imaging.

An x-ray computed microtomography system for specimen and small animal imaging was built and tested. The system used seventeen 48-microm-wide detector arrays (a charge coupled device camera) and helical acquisition techniques. Images were acquired using 540 rays/view and 400 views/2pi. The modulation transfer function (MTF) of the computed tomography images demonstrated 50 microm limiting resolution, with MTF > 10% for objects larger than 60 microm (approximately 8.3 cycles/mm). While soft tissue discrimination was compromised by a low signal-to-noise ratio, equine medullary bone core samples and the murine skeleton were well visualized. The incorporation of multiple detector arrays provided a 17-fold improvement in x-ray efficiency, which is a very important step toward improving the potential of microtomography as a scientific tool.     

Diagnostic x-ray spectra measurements using a silicon surface barrier detector

A silicon surface barrier detector is used to analyse diagnostic x-ray spectra. This detector, usually employed to detect charged particles, has a very low efficiency for x-ray. This characteristic is advantageous in overcoming experimental problems caused by the high fluence rates typical of diagnostic x-ray beams. The pulse height distribution obtained with silicon surface barrier detectors is very different from the true photon spectra because of the presence of escaped Compton photons and the fact that detection efficiency falls abruptly when photon energy increases. A detailed analysis of the spurious effects involved in detection is made by applying a Monte Carlo method. A stripping procedure is described for implementation on a personal computer. The validity of this method is finally tested by comparison with the experimental results obtained with a Ge detector. The spectra obtained with the Si detector are in fairly good agreement with the analogous spectra measured with a Ge detector. The advantages of using Si as opposed to Ge detectors in x-ray spectrometry can be summarised as: its simplicity of use, its greater economy for use in routine diagnostic x-ray spectroscopy and the possibility that the stripping procedure can be implemented on a personal computer.     

Direct measurement of mammographic x-ray spectra using a CdZnTe detector

Our purpose is to directly measure mammographic x-ray spectra with collimators and a low-efficiency CdZnTe detector developed recently and to find out the best fit response function of CdZnTe detector to correct the measured spectra. Photon spectra (target Mo or Rh) produced by a mammographic x-ray unit at 25-32 kV and 240 mAs (= 3 times of 80 mAs) and transmitted through 0.03 mm Mo or 0.025 mm Rh filter and object (0.1 mm Al to 0.8 mm Al phantoms) have been analyzed. Since detected spectra were distorted by the response of CdZnTe detector and did not present the true photon spectra, the correction was applied by the stripping procedure. The response function of detector used in this procedure has been determined by the evaluation of interactions (K-escape, coherent scattering, and Compton scattering processes) and incomplete charge collection calculated using the Monte Carlo method. We have used Kalpha1, Kalpha2, Kbeta1, Kbeta2 radiations of Cd, Zn, and Te, respectively and have used the weight function for the incomplete charge collection and have considered Compton scattering. The Monte Carlo simulations were continued by changing the important factors (mean path length of hole lambda(h), dead layer of the CZT crystal and weight factor Wq) of incomplete charge collection until the best fit response function was found out. Corrected photon spectra were compared with the mammographic x-ray spectral data of Bureau of Radiological Health (BRH) measured using a Ge detector. Attenuation curves of aluminum for 25-32 kV were calculated from the corrected photon spectra and compared with the attenuation curves measured using an ionization chamber. These results obtained using the CdZnTe detector agreed with the mammographic x-ray spectral data of BRH and attenuation curves obtained by the ionization chamber.     

Physical characteristics of a low-dose gas microstrip detector for orthopedic x-ray imaging

A new scanning slit gas detector dedicated to orthopedic x-ray imaging is presented and evaluated in terms of its fundamental imaging characteristics. The system is based on the micromesh gaseous structure detector and achieves primary signal amplification through electronic avalanche in the gas. This feature, together with high quantum detection efficiency and fan-beam geometry, allows for imaging at low radiation levels. The system is composed of 1764 channels spanning a width of 44.8 cm and is capable of imaging an entire patient at speeds of up to 15 cm/s. The resolution was found to be anisotropic and significantly affected by the beam quality in the horizontal direction, but otherwise sufficient for orthopedic studies. As a consequence of line-by-line acquisition, the images contain some ripple components due to mechanical vibrations combined with variations in the x-ray tube output power. The reported detective quantum efficiency (DQE) values are relatively low (0.14 to 0.20 at 0.5 mm(-1)) as a consequence of a suboptimal collimation geometry. The DQE values were found to be unaffected by the exposure down to 7 microGy, suggesting that the system is quantum limited even for low radiation levels. A system composed of two orthogonal detectors is already in use and can produce dual-view full body scans at low doses. This device could contribute to reduce the risk of radiation induced cancer in sensitive clientele undergoing intensive x-ray procedures, like young scoliotic women.     

Shimming with permanent magnets for the x-ray detector in a hybrid x-ray/ MR system

In this x-ray/MR hybrid system an x-ray flat panel detector is placed under the patient cradle, close to the MR volume of interest (VOI), where the magnetic field strength is approximately 0.5 T. Immersed in this strong field, several electronic components inside the detector become magnetized and create an additional magnetic field that is superimposed on the original field of the MR scanner. Even after linear shimming, the field homogeneity of the MR scanner remains disrupted by the detector. The authors characterize the field due to the detector with the field of two magnetic dipoles and further show that two sets of permanent magnets (NdFeB) can withstand the main magnetic field and compensate for the nonlinear components of the additional field. The ideal number of magnets and their locations are calculated based on a field map measured with the detector in place. Experimental results demonstrate great promise for this technique, which may be useful in many settings where devices with magnetic components need to be placed inside or close to an MR scanner.     

Physical characterization of a scanning photon counting digital mammography system based on Si-strip detectors

The physical performance of a scanning multislit full field digital mammography system was determined using basic image quality parameters. The system employs a direct detection detector comprised of linear silicon strip sensors in an edge-on geometry connected to photon counting electronics. The pixel size is 50 microm and the field of view 24 x 26 cm2. The performance was quantified using the presampled modulation transfer function, the normalized noise power spectrum and the detective quantum efficiency (DQE). Compared to conventional DQE methods, the scanning geometry with its intrinsic scatter rejection poses additional requirements on the measurement setup, which are investigated in this work. The DQE of the photon counting system was found to be independent of the dose level to the detector in the 7.6-206 microGy range. The peak DQE was 72% and 73% in the scan and slit direction, respectively, measured with a 28 kV W-0.5 mm Al anode-filter combination with an added 2 mm Al filtration.     

Optimal material discrimination using spectral x-ray imaging

Spectral x-ray imaging using novel photon counting x-ray detectors (PCDs) with energy resolving abilities is capable of providing energy-selective images. PCDs have energy thresholds, enabling the classification of photons into multiple energy bins. The extra energy information provided may allow materials such as iodine and calcium, or water and fat to be distinguishable. The information content of spectral x-ray images, however, depends on how the photons are grouped together. In this work, we present a model to optimize energy windows for maximum material discrimination. Multivariate statistics allows the confidence region of the correlated uncertainties to be mapped in the thickness space. Minimization of the uncertainties enables optimization of energy windows. Applications related to small animal imaging and breast imaging are considered.     

The x-ray light valve: a low-cost digital radiographic imaging system

In recent years new digital x-ray radiographic and fluoroscopic systems based on large-area flat-panel technology have revolutionized our capability of producing x-ray images. However, such imagers are extraordinarily expensive and their rapid image acquisition capability is not required for many applications such as radiography. Here we report a novel approach to achieve a high-quality digital radiographic system at a cost which is only a small fraction of competitive digital technologies. The results demonstrate that our proposed x-ray light valve system has excellent spatial resolution and adequate sensitivity compared to existing technologies.     

The photon sensitivity of a moderated activation neutron detector

A method is described for estimating the overresponse produced by neutrons generated in the hydrocarbon moderator of a neutron dosimetry system by photon reactions. It is shown that photon interactions in the moderator produce an apparent neutron fluence per unit absorbed dose of 1.40 X 10(6) n/cm2/Gy-x for a flattened 33-MV x-ray beam and 2.66 X 10(6) n/cm2/Gy-x for the same beam without the flattening filter in place. The photon response of the moderated activation neutron detector was found to decrease rapidly with end point energy of the x-ray beam in the range 33 to 18.7 MeV.     

Materials analysis using x-ray linear attenuation coefficient measurements at four photon energies

The analytical properties of an accurate parameterization scheme for the x-ray linear attenuation coefficient are examined. The parameterization utilizes an additive combination of N compositional- and energy-dependent coefficients. The former were derived from a parameterization of elemental cross-sections using a polynomial in atomic number. The compositional-dependent coefficients are referred to as the mixture parameters, representing the electron density and higher order statistical moments describing elemental distribution. Additivity is an important property of the parameterization, allowing measured x-ray linear attenuation coefficients to be written as linear simultaneous equations, and then solved for the unknown coefficients. The energy-dependent coefficients can be determined by calibration from measurements with materials of known composition. The inverse problem may be utilized for materials analysis, whereby the simultaneous equations represent multi-energy linear attenuation coefficient measurements, and are solved for the mixture parameters. For in vivo studies, the choice of measurement energies is restricted to the diagnostic region (approximately 20 keV to 150 keV), where the parameterization requires N >or= 4 energies. We identify a mathematical pathology that must be overcome in order to solve the inverse problem in this energy regime. An iterative inversion strategy is presented for materials analysis using four or more measurements, and then tested against real data obtained at energies 32 keV to 66 keV. The results demonstrate that it is possible to recover the electron density to within +/-4% and fourth mixture parameter. It is also a key finding that the second and third mixture parameters cannot be recovered, as they are of minor importance in the parameterization at diagnostic x-ray energies.     

Calibration model of a dual gain flat panel detector for 2D and 3D x-ray imaging

The continuing research and further development in flat panel detector technology have led to its integration into more and more medical x-ray systems for two-dimensional (2D) and three-dimensional (3D) imaging, such as fixed or mobile C arms. Besides the obvious advantages of flat panel detectors, like the slim design and the resulting optimum accessibility to the patient, their success is primarily a product of the image quality that can be achieved. The benefits in the physical and performance-related features as opposed to conventional image intensifier systems, (e.g., distortion-free reproduction of imaging information or almost linear signal response over a large dynamic range) can be fully exploited, however, only if the raw detector images are correctly calibrated and postprocessed. Previous procedures for processing raw data contain idealizations that, in the real world, lead to artifacts or losses in image quality. Thus, for example, temperature dependencies or changes in beam geometry, as can occur with mobile C arm systems, have not been taken into account up to this time. Additionally, adverse characteristics such as image lag or aging effects have to be compensated to attain the best possible image quality. In this article a procedure is presented that takes into account the important dependencies of the individual pixel sensitivity of flat panel detectors used in 2D or 3D imaging and simultaneously minimizes the work required for an extensive recalibration. It is suitable for conventional detectors with only one gain mode as well as for the detectors specially developed for 3D imaging with dual gain read-out technology.     

Brain tumor imaging using small-angle x-ray scattering tomography

We demonstrate high-resolution small-angle x-ray scattering computed tomography (SAXS-CT) of soft matter and soft tissue samples. Complete SAXS patterns over extended ranges of momentum transfer are reconstructed spatially resolved from volumes inside an extended sample. Several SAXS standard samples are used to quantitatively validate the method and demonstrate its performance. Further results on biomedical tissue samples (rat brains) are presented that demonstrate the advantages of the method compared to existing biomedical x-ray imaging approaches. Functional areas of the brains as well as tumor morphology are imaged. By providing insights into the structural organization at the nano-level, SAXS-CT complements and extends results obtainable with standard methods such as x-ray absorption tomography and histology.